The formation of arterial and venous (AV) branches must be exquisitely coordinated to generate proper AV circuitry essential for vascular function. The mechanism of AV coordination particularly that between paired, parallel arteries and veins is poorly understood. Our long-term objective is to elucidate the genetic program of mammalian AV circuitry. In the previous funding period, we reported that luminal sizes of the developing dorsal aorta (DA) and cardinal vein (CV) are synchronized. Notch signaling controls arterial specification and the allocation of both arterial and venous endothelial cells (ECs) into their respective vessels, thereby balancing the sizes of the developing DA and CV. We have also obtained preliminary data in mice suggesting that DA and CV formation is not initiated by pre-determined arterial and venous ECs, as previously thought. Instead, our work suggests a new step-wise model of mammalian parallel AV pair morphogenesis: the primitive unspecified artery assembles prior to the vein; followed by a phase of mixed AV identities in both vessels; finally the mixed ECs are segregated into uniformly-specified vessels with coordinated sizes. The specific aims of this grant are designed to test this new paradigm and to define the cellular mechanisms mediated by AV signaling in the morphogenesis of parallel AV pairs in mice. Our strategy is to take a cross- disciplinary approach including cutting-edge mouse genetics, cell lineage fate mapping, and imaging technologies. We recently built a custom 2-photon excited fluorescence microscope that is capable of imaging vasculature 1000 5m deep in living mouse tissue, achieving unprecedented resolution of previously inaccessible vascular structures. Aim 1 Examine Vascular Endothelial Growth Factor (VEGF)-mediated cell differentiation as a mechanism underlying heterogeneous arterial- and venous- fated ECs in the primordial DA (pDA) and CV (pCV). Aim 2 Examine cell segregation as a mechanism to sort venous-fated ECs in the pDA to the pCV. Aim 3 Determine the role of Notch signaling in coordinating the development of parallel artery and vein pairs. Aim 4 Determine the requirement of endothelial Notch1 and Coup-TFII in AV specification of adult parallel artery and vein pairs. Successful completion of this study will conceptually advance our knowledge of the morphogenesis and maintenance of parallel AV pairs, providing evidence regarding the origins of arteries and veins. Basic knowledge of the molecular mechanism of AV specification will inspire novel approaches to study blood vessel regeneration and vein graft engineering in disease settings. The combination of 2-photon high-resolution imaging with cutting-edge cell lineage tracing in living mouse embryos will be a major technological innovation for the field of mammalian vascular development at large.